Google検索の仕組み
https://support.google.com/webmasters/answer/70897?hl=ja
Googleクローラー
#(例)
html = open(url, 'User-Agent' => 'Googlebot/2.1')
doc = Nokogiri::HTML.parse(html, nil, "UTF-8")
Yahooクローラー
user_agent = 'Mozilla/5.0 (Windows NT 6.1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/28.0.1500.63 Safari/537.36'
html = open(url, "User-Agent" => user_agent